As with turbine blades and vanes formed from more conventional superalloy materials, CMC blades and vanes are primarily equipped with cavities and cooling voids to reduce weight, reduce centrifugal load, and reduce operating temperatures of the components. These features are typically formed in CMC components using a combination of removable and expendable tooling. Internal cooling channels are advantageous for cooling the both metal and CMC hot-gas path hardware as they reduce cooling flow requirements and thermal gradients/stress.
Silicon carbide (SiC)-based ceramic matrix composite (CMC) materials have been proposed as materials for certain components of gas turbine engines, such as the turbine blades, vanes, nozzles, and buckets. Various methods are known for fabricating SiC-based components, including Silicomp, melt infiltration (MI), chemical vapor infiltration (CVI), polymer inflation pyrolysis (PIP), and oxide/oxide processes. Though these fabrication techniques significantly differ from each other, each involves the use of hand lay-up and tooling or dies to produce a near-net-shape part through a process that includes the application of heat at various processing stages.
Current fabrication methods for forming internal passages or cavities in CMC components use materials that need to be “melted” out or removed from the internal passage during the burn-out cycle. Forming CMC components with a cavity includes a number of steps, including using pre-forms. First, a plurality of ceramic plies, some of which can include reinforcing material or are pre-impregnated with matrix, are laid up on a mandrel or mold in a pre-determined pattern to provide desired final or near-net-shape and desired mechanical properties of components. The mandrel is generally selected from materials such as tin, vinyl, or other meltable materials. The laid-up plies may be pre-impregnated (pre-preg) with matrix material, such as SiC or impregnated with matrix after lay-up of the plies. Prior to densification of the CMC pre-form, the mandrel is removed through a burn-out cycle. In the burn-out cycle, the CMC pre-form is inverted and the mandrel forming materials, such as tin, vinyl, or other meltable materials, are melted out through the open tip of the pre-form CMC leaving an open tip area.